1. Field of the Invention
The field of the invention is that of devices for generating current pulses, also known as charging pumps.
To be more precise, the invention concerns a low-noise device for generating current pulses.
The invention applies in particular, although not exclusively, to generating low-noise current pulses for a phase comparator included in a synthesizer of a radiocommunication terminal.
2. Description of the Prior Art
A device of the above type generally comprises current pulse generator means cooperating with polarizer means. The polarizer means supply a polarization voltage to the current pulse generator means.
There are two major families of current pulse generator devices according to the basic structure formed by the polarizer means and the current pulse generator means. The basic structure is either of the switched current mirror type or of the cascode current source type.
A switched current mirror current pulse generator device generally generates current pulses by switching switch means .phi. and .phi. (see FIG. 1) at a frequency depending on the required frequency of the pulses. A current pulse is generated in two successive phases. In a first phase the switch means .phi. are closed and the switch means .phi. are open to cause current to flow to the output (at the source of the transistor of the current pulse generator means). In this first type of device the algebraic value of the current corresponds to that of the current leaving the source of the transistor of the polarizer means. In a second phase the switch means .phi. are open and the switch means .phi. are closed to prevent the flow of current. To generate a stream of current pulses the two phases are repeated at a frequency equal to that required for the output current pulses and corresponding to that imposed by the opening and closing of the switch means.
The cascode current source current pulse generator device generates current pulses by switching the switch means .phi. alone (see FIG. 2) at a frequency depending on the required frequency of the pulses. In a first phase the switch means .phi. are open allowing current to flow at the output (at the source of the cascode transistor). During a second phase the switch means .phi. are closed to prevent the flow of current at the output. To generate a stream of current pulses the two phases are repeated, as in the first type of device, at a frequency equal to the required frequency of the output current pulses and corresponding to that imposed by the switching.
However, the polarizer means of the above two types of current pulse generator devices generate undesirable noise. The components of the polarizer means do not exhibit ideal behavior and therefore generate noise. Such noise (in particular that due to the current source or sources and that due to the imperfections of the various components of the polarizer means) is recovered by the current pulse generator means and also transmitted on the output side of those means, possibly amplified.
Also the polarizer means are subject to power supply rejection that is equally problematic. Insufficient power supply rejection disturbs the operation of most of the components on the output side of the polarizer means.
A prior art solution aimed at reducing noise consists in isolating the power supplies of the system using them. A solution of this kind is not satisfactory because it is not readily compatible with an integrated system. There remain high levels of coupling within the system, generating interference.
Another prior art solution is to use a low charge injection logic circuit. This solution is also relatively disadvantageous because it is not readily compatible with a system whose overall size is to be reduced. A low charge injection logic circuit of the above kind necessitates the addition of new components.
One objective of the present invention is to alleviate the various drawbacks of the prior art.
To be more precise, one objective of the present invention is to provide a current pulse generator device which can be integrated into the system using it.
An additional objective of the invention is to provide a device of the above kind that strongly reduces or even eliminates the various kinds of noise generated by the polarizer means.
Another objective of the invention is to provide a device of the above kind that is simple to manufacture.
An additional objective is to provide a device of the above kind optimizing the costs of manufacture.